The switched current (SI) technique is a relatively new analog sampled data signal processing technique, that fully exploits digital CMOS technology, see for example "Switched currents, an analogue technique in digital technology" by C. Toumazou, J. B. Hughes and N. C. Battersby, Peter Peregrinius Ltd, 1993. The ultimate performance of the SI circuits is primarily determined by the transconductance g.sub.m of an MOS transistor and the capacitance Cg seen by its gate. Although a high speed operation (around 100 Mhz) is possible, the performance of speed and accuracy is limited by the technique itself. In order to have high accuracy, large Cg is usually required, since the clock feedthrough error is inversely proportional to Cg. Therefore, increasing g.sub.m is the only way to increase the speed. With the same bias current, the transconductance of an MOS transistor is considerably lower than that of a bipolar transistor. Therefore, utilization of the bipolar transistor could increase the speed and/or improve the accuracy. BiCMOS technology opens the possibility of using both MOS and bipolar transistors.
A technique has been proposed for a high speed sampled-data signal processing in BiCMOS, see for example "A new BiCMOS technique for very fast discrete time signal processing" by P. Shak and C. Toumazou in Proc. 1995 International Symposium on Circuits and Systems pp. 323-326. It breaks the limitation of g.sub.m /Cg of the SI circuits by utilizing bipolar transistors. It first converts a current to a voltage by a transresistor and then converts the voltage to a current by a transconductor. The voltage is sampled and held at the input of the transconductor, whose input device is an MOS transistor. However, the conversion accuracy is determined by the absolute value of the components. For example, the resistor determines the transresistance value, and the transistor size and the operation condition determine the transconductance value. Therefore the technique is sensitive to process variation and another drawback is its complexity.
Patent applications have been filed by John B. Hughes of Philips, U.K. on a technique, which was referred to as the switched-current technique, see for example EP 89203067.7, Apr. 12, 1989. All these applications were concentrated on the technique for a digital CMOS process.